1. Field of the Invention
The present invention relates to a technology for producing a gallium nitride (GaN) crystal and a GaN substrate.
2. Description of the Related Art
Indium gallium aluminum (InGaAln)-based devices (group-III nitride semiconductor) are popularly used as ultraviolet light sources, purple light sources, blue light sources, or green light sources. Such group-III nitride semiconductor devices are typically formed on a substrate made of sapphire or silicon carbide (SiC) by metalorganic chemical vapor deposition (MOCVD) method or molecular beam epitaxy (MBE) method.
However, there is considerable difference between the coefficients of thermal expansion and the lattice constants of the substrate, which is made of sapphire or SiC, and the group-III nitride semiconductor. Such differences in the physical properties of the substrate and the group-III nitride semiconductor result in causing crystal defects (imperfections) in the group-III nitride semiconductor. If the group-III nitride semiconductor has crystal defects, performance of the device, such as a light-emitting device, made from the group-III nitride semiconductor degrades, e.g., device lifetime is shortened, large driving power is necessary, and the like.
Some of the conventional light emitting devices are made from a conducting substrate so that it was possible to obtain an electrode from such a conducting substrate. However, because sapphire is insulating, it is difficult to obtain an electrode from such an insulating substrate, requiring obtaining an electrode from a group-III nitride semiconductor. To obtain an electrode from a group-III nitride semiconductor, a device needs to be large, resulting in increasing necessary costs. Furthermore, if the device size increases, the substrate may warp because of the differences in the physical properties of the sapphire substrate and the group-III nitride semiconductor.
In a group-III nitride semiconductor device formed on a sapphire substrate, separation of chips by use of cleavages is difficult, so that it is difficult to obtain a desired resonator facet for a laser diode (LD). For counteracting above problems, in one approach, the resonator facet is formed by the dry etching method, or the method of separating a sapphire substrate in a manner similar to cleavage after polishing the sapphire substrate to make it as thick as 100 micrometers (μm) or less. In this approach, however, it is difficult to form a resonator facet and to conduct chip separation in a single process, unlike a process for forming a conventional LD. As a result, manufacturing costs increase due to necessity of extra processes.
Another approach for reducing crystal defects is to selectively grow a group-III nitride semiconductor on a sapphire substrate in a longitudinal direction. Although occurrence of crystal defect can be reduced in this approach, it is still difficult to solve problems related to insulation properties or cleavage of a sapphire substrate.
In still another approach, the substrate is made of gallium nitride (GaN). In other words, the substrate is made of the same material as the crystal grown on the substrate. For example, Japanese Patent No. 3788037 discloses a technology for producing a GaN substrate. The GaN substrate is produced by growing a GaN crystal on a gallium arsenic (GaAs) substrate by hydride vapor phase epitaxy (HVPE) method, and then, slicing grown GaN crystal.
However, the GaN crystal is formed using coalescence and bending of dislocation to reduce occurrence of dislocation. Therefore, it is difficult to obtain desired flatness on a surface of a GaN crystal, resulting in making it difficult to produce a bulked GaN crystal in desired quality with less defect density.
Furthermore, when a GaN crystal is polished and sliced, mechanical damage and etching easily occurs in a crystal grain boundary or a region of dislocation. Thus, it is difficult to produce a wafer in desired quality with preferable flatness of its surface.